[noun] A preparation of a weakened or killed pathogen, such as a bacterium or virus, or of a portion of the pathogen's structure that is used to stimulate immunity to a particular disease or antibody production.

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[noun] The number of single bonds an atom can form, also measured as the number of hydrogen atoms that typically bond to an atom of an element. For example, in H2O, oxygen has a valence of two; in CH4, carbon has a valence of four.

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[noun] Electrons that can be actively involved in chemical change; usually electrons in the shell with the highest value of n (electrons in the "outermost" shell of an atom). For example, sodium's ground state electron configuration is 1s2 2s2 2p6 3s1; the 3s electron is the only valence electron in the atom. Valence electrons determine the chemical properties of an atom and are the only electrons that participate in chemical bonding.

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[verb] To establish the soundness or truth of something, often using an independent means of checking results. Validation of data, models, statistical analyses, etc. is a key component of the process of science.

[noun] A general term for all attractive forces that occur between electrically neutral molecules. Van der Waals forces include dipole-to-dipole interactions, hydrogen bonds, and other intermolecular forces.

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[noun] In math, an expression that can be assigned any set of values. Variables are written as symbols, such as x, y or z, representing unspecified quantities or members of a set.

In science, the term refers to a condition or parameter that may be manipulated, fixed, measured, or observed in the course of scientific research. There are usually three categories of variables in science experiments; control, independent (experimental) and dependent (response). For more information, see our module Experimentation in Scientific Research.

[person] (born 1946) American geneticist and entrepreneur, renown for leading the charge to sequence the human genome. While a researcher at the National Institutes of Health (NIH), Venter developed a technique called expressed sequence tags (ESTs) that made rapid sequencing possible. In 1998, he started Celera Genomics, a privately funded company, to pursue genome projects.

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[noun] A small, self-enclosed sac inside of cells containing fluid and other material. The vesicles organize the cell's metabolism, help store enzymes in the cell, and serve a transport function by moving molecules between locations within the cell. Vesicles are also used as containers to isolate damaging chemical reactions from occurring in the cytosol.

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[person] English geophysicist, born in London (1939-). With his advisor, Drummond Matthews, Vine wrote "Magnetic Anomalies Over Ocean Ridges," presenting ideas that helped the theory of plate tectonics gain widespread recognition and adherents. Vine also researched the history of Earth's magnetic field and studied the electrical conductivity of the lower continental crust.

[person] Polish biologist, born in Scievelbein, Pommerania (today Swindin, Poland) (1821-1902). In 1858 he published his best-known work, The Foundation of Cellular Pathology on the Basis of a Physiological and Pathological Understanding of Tissue, formalizing the cell theory.

[noun] An ultramicroscopic infectious agent that can only replicate within the cells of living hosts, mainly bacteria, plants, and animals. Viruses have a very basic structure, consisting of an RNA or DNA core surrounded by a protein coat. More complex viruses also have an envelope surrounding the core and protein coat. Viruses have a wide range of effects on their hosts, from nearly harmless (such as toxoplasmosis in cats) to fatal (such as Ebola in humans).

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[verb] The measurement of a fluid's resistance to shear or flow. Highly viscous fluids resist motion due to their molecular composition that produce internal friction. Honey, for example, is a fluid with a high viscosity. Low viscosity fluids, conversely, flow easily because their molecular composition results in very little friction.

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[noun] The distance the human eye can see through a given medium, such as air or water, under certain environmental conditions.

In aquatic sciences, visibility is usually measured in meters with a Secchi disc. Visibility in water can be an indication of water turbulence. As water moves within a given area, it stirs up sediment and nutrients that have settled onto the area’s bottom – an action similar to shaking a snow globe. Visibility can also be an indication of how much productivity is occurring in the aquatic system. For example, all other variables being equal, a pond that is rich in plant and animal life will likely have a lower visibility than a pond that has very little life. This is because there are more plankton and particulate matter in the water column of the pond rich in life – similar to the difference between looking down a crowded street and looking down the same street when it is empty of people.

[person] (November 15, 1871 – October 11, 1962) An Austrian agronomist whose research focused on developing disease-resistant crops, such as wheat-rye and oat hybrids. In 1900, von Seysenegg published a paper presenting his rediscovery of Gregor Mendel's work on genetics.